Traditionally electricity system stability has been provided by spinning reserve, which is spare generating capacity available to the system should unexpected loads, such as a large fault, threaten system stability. Spinning reserve is expensive as it does not produce any revenue for the generating company but it is required for system stability.
Attempts have been made to supplement the stabilizing effect of spinning reserve with fixed frequency relays fitted to major loads or sub-stations within a network. These relays disconnect their loads if the system frequency (nominally for example 50 hertz) drops too much indicating a system disturbance such as a major fault. The loads are then reconnected when the system has stabilised. The major disadvantage of this arrangement is its inflexibility. The dynamic nature of the loading on a network means that over time the network will require different stabilizing tactics which cannot be accommodated satisfactorily by the fixed parameters of these frequency relays.